1. Field of the Invention
The present invention relates to a data transmission method for ocean acoustic tomography and, more particularly, to a system by which data obtained with a wide area ocean acoustic tomography system is appropriately transmitted to a ground station via a mooring repeater and an artificial satellite.
2. Description of the Prior Art
A sound wave travelling in the sea contains information about physical properties of the ocean. Accordingly, clarification of the influence of physical properties of the ocean on acoustic signals will make it possible to derive therefrom information about the ocean. The principle of the ocean acoustic tomography is based on a simple fact that the information about physical properties of the ocean is contained in the structure of the sound field which travels in the sea. Since the sound speed in the sea depends largely upon the temperature, salinity and density (pressure) of seawater, the travel time of the sound wave reflects the states of these physical parameters. Accordingly, the states of the physical parameters of the ocean (i.e. the ocean structure) can be presumed by an inversion method or inverse calculation from the travel time of the sound wave.
The sound speed in the sea is related to the temperature, salinity and pressure of seawater. For example, according to "Acoustical Oceanography Principle and Applications", Clarence S. Clay and Herman Medwin, 1972, the sound speed C (m/s) depends upon the following equation: EQU C=1449.2+4.6T-0.055T.sup.2 +0.00029T.sup.3 +(1.34-0.01T) (S-35)+1.58.times.10.sup.-6 Pa
where T is water temperature (C.degree.), S salinity (%), and Pa gage pressure (N/m.sup.2).
According to another publication ("Introduction to Acoustical Engineering", Revised Edition, published by Nikkan Kogyo Shimbunsha), EQU C=1410+4.21t-0.037t.sup.2 +1100S+0.018d
where t is water temperature (C.degree.), S salinity (Kg weight of salt contained in 1 Kg of seawater), and d is the depth of water (m).
Munk and Wunsch have proposed an ocean acoustic tomography system which utilizes acoustic transmission. With this system, meso-scale ocean variability covering a spatical expanse over several hundred kilometers in the ocean can be presumed by measuring the travel time of each of acoustic pulses which are transmitted between acoustic transducers T and acoustic receivers R disposed at observation points in the ocean as shown in FIG. 1. That is, oceanic phenomena such as a front 1 and an eddy 2 in the ocean space can be presumed. A sound speed profile in seawater has a sound speed minimum layer at a certain depth. Namely, since the salinity of seawater in a certain sea area can be regarded as constant, only the temperature and pressure of seawater need to be taken into account. When the pressure is fixed, higher the water temperature, the higher the sound speed. When the water temperature is fixed, the higher the pressure, the higher the sound speed. As the depth of water increases, the water temperature decreases and the pressure increases. In medium latitudes the sound speed minimum layer exists at a depth of 1000 m or so. Since a sound wave travels while being refracted toward layers of lower sound speeds, it travels along the sound speed minimum layer. FIG. 2 shows, by way of example, the sound speed profile and sound ray paths (obtained by simulation) between a transmitter and a receiver disposed 300 Km apart. In a sea area more than 4000 m deep, sound can be propagated stably over a long distance without reflection by the sea bottom and the sea surface.
FIGS. 3A and 3B show, by way of example, transmission loss-travel time characteristics of acoustic pulses travelling between a transmitter and a receiver disposed 100 Km apart and sound ray paths (obtained by simulation) between them in the case where the pulse frequency is 220 Hz and the acoustic source and receiver are disposed at depths of 1.35 and 1.75 Km, respectively.
In 1981 an experiment was conducted in the southern part of the North Atlantic (see "OCEAN ACOUSTIC TOMOGRAPHY: A New Measuring Tool", OCEANUS 25-2, pp. 12-21, 1982, for example). In this experiment a sound velocity distribution was obtained in a sea area 300 Km around. FIG. 4 schematically shows the positional relationship between acoustic transducers and receivers used in the experiment. In this instance the acoustic transducers transmit pulse signals of a 200 to 300 Hz frequency and the transmitted data is received by (and recorded in) the acoustic receivers and is analyzed after recovery thereof. That is, this conventional system involves the necessity of recovering the receivers from the sea prior to the analysis of the measured data. Further, since no data is accessible during measurement, it is impossible to know parameters such as the direction of an ocean current, water temperature, etc. as well as the operating conditions (including troubles) of the acoustic transducers and receivers.
Moreover, there has been proposed an underwater data collecting method according to which a signal sent from a submersible buoy (a submersible drifting buoy) is received by a repeating buoy and thence transmitted in the form of an electric wave (Japanese Pat. Pub. Disc. No. 92153/78). However, the submersible buoy for use in this method is a drifting buoy which is designed to stay at a certain depth of water without employing a mooring rope, and data available by this method is not acoustic tomography data.